Pre-press color proofing is a procedure that is used by the printing industry for creating representative images of printed material, without the high cost and time that is required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy the requirements of customers, resulting in a large loss of profits. By utilizing pre-press color proofing time and money can be saved.
One such commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,268,708 is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying a sufficient amount of thermal energy to the dye donor material to form an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, a lathe bed scanning subsystem (which includes a lathe bed scanning frame, a translation drive, a translation stage member, a printhead, and a vacuum imaging drum), and thermal print media and dye donor material exit transports.
The operation of the above image processing apparatus comprises metering a length of the thermal print media (in roll form) from the material assembly or carousel. The thermal print media is then measured and cut into sheet form of the required length, transported to the vacuum imaging drum, registered, wrapped around and secured onto the vacuum imaging drum. Next a length of dye donor material (in roll form) is also metered out of the material supply assembly or carousel, then measured and cut into sheet form of the required length. It is then transported to and wrapped around the vacuum imaging drum, such that it is superposed in the desired registration with respect to the thermal print media (which has already been secured to the vacuum imaging drum).
After the dye donor material is secured to the periphery of the vacuum imaging drum, the scanning subsystem or write engine provides the scanning function. This is accomplished by retaining the thermal print media and the dye donor material on the spinning vacuum imaging drum while it is rotated past the printhead that will expose the thermal print media. The translation drive then traverses the printhead and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum. These movements combine to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the dye donor material is then removed from the vacuum imaging drum. This is done without disturbing the thermal print media that is beneath it. The dye donor material is then transported out of the image processing apparatus by the dye donor material exit transport. Additional dye donor materials are sequentially superposed with the thermal print media on the vacuum imaging drum, then imaged onto the thermal print media as previously mentioned, until the intended image is completed. The completed image on the thermal print media is then unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport.
The material supply assembly comprises a carousel assembly mounted for rotation about its horizontal axis on bearings at the upper ends of vertical supports. The carousel comprises a vertical circular plate having in this case six (but not limited to six) material support spindles. These support spindles are arranged to carry one roll of thermal print media, and four rolls of dye donor material to provide the four primary colors used in the writing process to form the intended image, and one roll as a spare or for a specialty color dye donor material (if so desired). Each spindle has a feeder assembly to withdraw the thermal print media or dye donor material from the spindles to be cut into a sheet form. The carousel is rotated about its axis into the desired position, so that the thermal print media or dye donor material (in roll form) can be withdrawn, measured, and cut into sheet form of the required length, and then transported to the vacuum imaging drum.
The scanning subsystem or write engine of the lathe bed scanning type comprises a mechanism that provides the mechanical actuators, for the vacuum imaging drum positioning and motion control to facilitate placement, loading onto, and removal of the thermal print media and the dye donor material from the vacuum imaging drum. The scanning subsystem or write engine provides the scanning function by retaining the thermal print media and dye donor material on the rotating vacuum imaging drum, which generates a once per revolution timing signal to the data path electronics as a clock signal; while the translation drive traverses the translation stage member and printhead axially along the vacuum imaging drum in a coordinated motion with the vacuum imaging drum rotating past the printhead. This is done with positional accuracy maintained, to allow precise control of the placement of each pixel, in order to produce the intended image on the thermal print media.
The lathe bed scanning frame provides the structure to support the vacuum imaging drum and its rotational drive. The translation drive with a translation stage member and printhead are supported by two translation bearing rods that are substantially straight along their longitudinal axis and are positioned parallel to the vacuum imaging drum and lead screw. Consequently, they are parallel to each other therein forming a plane, along with the vacuum imaging drum and lead screw. The translation bearing rods are, in turn, supported by outside walls of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine. The translation bearing rods are positioned and aligned therebetween, for permitting low friction movement of the translation stage member and the translation drive. The translation bearing rods are sufficiently rigid for this application, so as not to sag or distort between the mounting points at their ends. They are arranged to be as exactly parallel as is possible with the axis of the vacuum imaging drum. The front translation bearing rod is arranged to locate the axis of the printhead precisely on the axis of the vacuum imaging drum with the axis of the printhead located perpendicular, vertical, and horizontal to the axis of the vacuum imaging drum, The translation stage member front bearing is arranged to form an inverted "V" and provides only that constraint to the translation stage member. The translation stage member with the printhead mounted on the translation stage member, is held in place by only its own weight. The rear translation bearing rod locates the translation stage member with respect to rotation of the translation stage member about the axis of the front translation bearing rod. This is done so as to provide no over-constraint of the translation stage member which might cause it to bind, chatter, or otherwise impart undesirable vibration or jitters to the translation drive or printhead during the writing process causing unacceptable artifacts in the intended image. This is accomplished by the rear bearing which engages the rear translation bearing rod only on a diametrically opposite side of the translation bearing rod on a line perpendicular to a line connecting the centerlines of the front and rear translation bearing rods.
The translation drive is for permitting relative movement of the printhead by synchronizing the motion of the printhead and stage assembly such that the required movement is made smoothly and evenly throughout each rotation of the drum. A clock signal generated by a drum encoder provides the necessary reference signal accurately indicating the position of the drum. This coordinated motion results in the printhead tracing out a helical pattern around the periphery of the drum. The positional error of the printhead can be characterized and is shown to be periodic with a frequency that is 4 times the frequency of a composite current waveform that drives a stepper motor.
With the previously discussed color proofing system, the translation drive motion is obtained using a DC servo motor with a feedback encoder. The DC servo motor rotates a lead screw that is aligned generally in parallel with the axis of the vacuum imaging drum. The printhead is placed on the translation stage member in a "V" shaped groove, which is formed in the translation stage member, which is in precise positional relationship to the bearings for the front translation stage member supported by the front and rear translation bearing rods. The translation bearing rods are positioned parallel to the vacuum imaging drum, so that the translation stage member automatically adopts the preferred orientation with respect to the surface of the vacuum imaging drum.
The printhead is selectively locatable with respect to the translation stage member; thus it is positioned with respect to the vacuum imaging drum surface. By adjusting the distance between the printhead and the vacuum imaging drum surface, as well as the angular position of the printhead about its axis using adjustment screws, an accurate means of adjustment for the printhead is provided. Extension springs provide the load against these two adjustment means.
The translation stage member and printhead are attached to a rotatable lead screw (having a threaded shaft) by a drive nut and coupling. The coupling is arranged to accommodate misalignment of the drive nut and lead screw so that only rotational forces and forces parallel to the lead screw are imparted to the translation stage member by the lead screw and drive nut. The lead screw rests between two sides of the lathe bed scanning frame of the lathe bed scanning subsystem or write engine, where it is supported by deep groove radial bearings. At the drive end the lead screw continues through the deep groove radial bearing, through a pair of spring retainers, that are separated and loaded by a compression spring to provide axial loading, and to a DC servo drive motor and encoder. The DC servo drive motor induces rotation to the lead screw moving the translation stage member and printhead along the threaded shaft as the lead screw is rotated. The lateral directional movement of the printhead is controlled by switching the direction of rotation of the DC servo drive motor and thus the lead screw.
The printhead includes a plurality of laser diodes which are coupled to the printhead by fiber optic cables which can be individually modulated to supply energy to selected areas of the thermal print media in accordance with an information signal. The printhead of the image processing apparatus includes a plurality of optical fibers coupled to the laser diodes at one end and the other end to a fiber optic array within the printhead. The printhead is movable relative to the longitudinal axis of the vacuum imaging drum. The dye is transferred to the thermal print media as the radiation, transferred from the laser diodes by the optical fibers to the printhead and thus to the dye donor material is converted to thermal energy in the dye donor material.
The printhead writes its image as a swath comprising a plurality of laser diode signals, where this swath is written in a helical pattern in coordination with the rotation of the vacuum imaging drum. To minimize possible imaging anomalies due to frequencies of dot patterns and the characteristics of the image writing hardware, it is advantageous to be able to write the image with a variable number of lasers. U.S. Pat. No. 5,329,297, the subject matter of which is herein incorporated by reference, describes this problem in detail and discloses how this can be achieved with the existing system. Briefly, this is accomplished by disabling lasers on the outer periphery of the swath and changing the timing of printhead movement across the vacuum imaging drum to correspond to the changed swath width.
The vacuum imaging drum is cylindrical in shape and includes a hollowed-out interior portion. The vacuum imaging drum further includes a plurality of holes extending through its housing for permitting a vacuum to be applied from the interior of the vacuum imaging drum for supporting and maintaining the position of the thermal print media and dye donor material as the vacuum imaging drum rotates. The ends of the vacuum imaging drum are enclosed by cylindrical end plates. The cylindrical end plates are each provided with a centrally disposed spindle which extends outwardly through support bearings and are supported by the lathe bed scanning frame. The drive end spindle extends through the support bearing and is stepped down to receive a DC drive motor rotor which is held on by means of a nut. A DC motor stator is stationarily held by the lathe bed scanning frame member, encircling the armature to form a reversible, variable speed DC drive motor for the vacuum imaging drum. At the end of the spindle an encoder is mounted to provide the timing signals to the image processing apparatus. The opposite spindle is provided with a central vacuum opening, which is in alignment with a vacuum fitting with an external flange that is rigidly mounted to the lathe bed scanning frame. The vacuum fitting has an extension which extends within but is closely spaced from the vacuum spindle, thus forming a small clearance. With this configuration, a slight vacuum leak is provided between the outer diameter of the vacuum fitting and the inner diameter of the opening of the vacuum spindle. This assures that no contact exists between the vacuum fitting and the vacuum imaging drum which might impart uneven movement or jitters to the vacuum imaging drum during its rotation.
The opposite end of the vacuum fitting is connected to a high-volume vacuum blower which is capable of producing 50-60 inches of water (93.5-112.2 mm of mercury) at an air flow volume of 60-70 cfm (28.368-33.096 liters per second). This provides the vacuum to the vacuum imaging drum to support the various internal vacuum levels of the vacuum imaging drum required during the loading, scanning and unloading of the thermal print media and the dye donor materials to create the intended image. With no media loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 10-15 inches of water (18.7-28.05 mm of mercury). With just the thermal print media loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 20-25 inches of water (37.4-46.75 mm of mercury); this is the level required when a dye donor material is removed so that the thermal print media does not move, otherwise color to color registration will not be maintained. With both the thermal print media and dye donor material completely loaded on the vacuum imaging drum the internal vacuum level of the vacuum imaging drum is approximately 50-60 inches of water (93.5-112.2 mm of mercury) in this configuration.
The task of loading and unloading the dye donor materials onto and off from the vacuum imaging drum requires precise positioning of the thermal print media and the dye donor materials. The lead edge positioning of dye donor material must be accurately controlled during this process. Existing image processing apparatus designs, such as that disclosed in the above-mentioned commonly assigned U.S. patent, employs a multi-chambered vacuum imaging drum for such lead-edge control. One appropriately controlled chamber applies vacuum that holds the lead edge of the dye donor material. Another chamber, separately valved, controls vacuum that holds the trail edge of the thermal print media, to the vacuum imaging drum. With this arrangement, loading a sheet of thermal print media and dye donor material requires that the image processing apparatus feed the lead edge of the thermal print media and dye donor material into position just past the vacuum ports controlled by the respective valved chamber. Then vacuum is applied, gripping the lead edge of the a dye donor material against the vacuum imaging drum surface.
Unloading the dye donor material or the thermal print media (to discard the used dye donor material or to deliver the finished thermal print media to an output tray) requires the removal of vacuum from these same chambers so that an edge of the thermal print media or the dye donor material are freed and project out from the surface of the vacuum imaging drum. The image processing apparatus then positions an articulating skive into the path of the free edge to lift the edge further and to feed the dye donor material, to a waste bin or an output tray.
The sheet material exit transports include a dye donor material waste exit and the imaged thermal print media sheet material exit. The dye donor material exit transport comprises a waste dye donor material stripper blade disposed adjacent the upper surface of the vacuum imaging drum. In an unload position, the stripper blade is in contact with the waste dye donor material on the vacuum imaging drum surface. When not in operation, the stripper blade is moved up and away from the surface of the vacuum imaging drum. A driven waste dye donor material transport belt is arranged horizontally to carry the waste dye donor material, which is removed by the stripper blade from the surface of the vacuum imaging drum to an exit formed in the exterior of the image processing apparatus. A waste bin for the waste dye donor material is separate from the image processing apparatus. The imaged thermal print media sheet material exit transport comprises a movable thermal print media sheet material stripper blade that is disposed adjacent to the upper surface of the vacuum imaging drum. In the unload position, the stripper blade is in contact with the imaged thermal print media on the vacuum imaging drum surface. In the inoperative position, it is moved up and away from the surface of the vacuum imaging drum. A driven thermal print media sheet material transport belt is arranged horizontally to carry the imaged thermal print media removed by the stripper blade from the surface of the vacuum imaging drum. It then delivers the imaged thermal print media with the intended image formed thereon to an exit tray in the exterior of the image processing apparatus.
Although the presently known and utilized image processing apparatus is satisfactory, it is not without drawbacks. The DC servo motor that is used to drive the lead screw requires feedback control signals from an expensive, high-precision encoder. With the present arrangement, control circuitry must accept the encoder signal as input and process this feedback signal to obtain the correct output signal for driving the DC servo motor. The need for these added components increases the cost and design complexity of the image processing apparatus.
As an alternative method for providing precise rotational positioning, a stepper motor can be employed. Stepper motors provide precise rotational motion that can be used to rotate a lead screw device in order to provide precise linear motion. The stepper motor has a shaft motion characterized by the capability to achieve discrete angular movements of uniform magnitude based on its input signal. In its simplest implementation, this type of motor is driven by a sequentially switched DC power supply that provides square-wave current pulses rather than analog current values.
Internally, the stepper motor uses magnetic attraction and repulsion of a rotor in discrete steps so that the rotor takes an angular orientation at some integral multiple of a divisor angle that is based on the number and position of stator teeth and on rotor characteristics. To achieve this controlled motion, the stepper motor has two separate windings (A and B). The drive components for the stepper motor coordinate the timing of current to each set of windings so that different internal stator poles have different magnetic states for each rotor position. In a "full step current, 2-phase on" mode, windings A and B are independently energized in one of two discrete current levels, at full current. This arrangement provides highly precise positioning for most stepper motors to, typically, 400 steps per rotation. With 400 steps per rotation, each step moves the rotor 0.9 degrees.
For an image processing apparatus, however, finer resolution than this typical 400 steps per revolution is required. To achieve finer resolution from the stepper motor and lead screw design itself, there would be significant physical and cost limitations. For example, using a lead screw having finer resolution is more costly and requires that the drive motor accelerate and run at faster speeds than may be practical for rapid starting and stopping. This requirement for higher speeds also complicates synchronization between the printhead traversal subsystem and the vacuum drum motor. To overcome this and other limitations, the stepper motor can be used in a microstepping mode. This uses the fact that variable amounts of current through stator windings in turn vary the amount of magnetic force in the stator pole. This allows the rotor to take intermediate angular positions, between the discrete "step" positions described earlier.
In a microstepping mode, the phase current exhibits a voltage-time relationship with discrete steps such that the composite waveform is sinusoidal. With microstepping, the A and B phases are substantially two sine waves with 90 degrees phase shift from each other. Since the rotor position adjusts in some proportion to the magnetic force from stator windings, this allows the rotor to take intermediate positions. This arrangement gives the stepper motor many times the positioning resolution of discrete stepping using square wave current input. Typical upper range achievable using microstepping: 500 microsteps per step. For a motor with 400 steps per revolution, for example, this would allow 200,000 microsteps per revolution.
The tradeoffs with microstepping include variable torque, since different levels of current are flowing for each different position. In addition, since stator windings are energized at some intermediate current level, rather than at full current, rotor position is not as stable as with full step mode. Hence, the accuracy of each microstep is not as precise as is accuracy for full steps. Typically, feedback loops are employed to improve positioning as compensation for this loss of positional accuracy when using microstepping. However, feedback loops require costly design effort and precision feedback components.
The mechanism for printhead positioning in an image processing apparatus must overcome the inherent inaccuracy in microstepping, as described above. This presents particular difficulty for the process of synchronizing printhead positioning at the beginning of each swath. Any additive error that accumulates over the length of the image may cause sizing problems, banding, or other objectionable image anomalies. (A method for handling the above problem is disclosed commonly assigned copending application entitled "Programmable Gearing Control of a Leadscrew for a Printhead Having a Variable Number of Channels" Attorney Docket No. 78184). A further complication that can cause image anomalies is swath-to-swath error that is a result of stepper motor inaccuracy when running in microstepping mode. The periodic behavior of stepper motor positional error can cause visible moire patterns, "beating", or other imaging anomalies on the final image. Each rotation of the vacuum imaging drum writes one swath. With periodic positional error sufficiently out of phase from one swath to the next, the resulting swath pattern can cause objectionable imaging effects U.S. Pat. No. 5,278,578 describes how the error frequency, swath-to-swath, can affect the output image by producing a "beat" frequency that can vary depending on the halftone dot resolution of the image.
There are a number of factors that determine the phase relationship of the periodic positioning error, swath-to-swath. Chiefly, these are: the image resolution, the number of channels that write each swath, the thread pitch of the lead screw, and the stepper motor speed required. Of these factors, the image resolution is typically fixed to one value. Stepper motor speed must be selected within a practical range, considering timing and start-stop requirements. Ideally, the image processing apparatus should support a variable number of channels for the image quality reasons described in the above-cited U.S. Pat. No. 5,329,297.
The use of microstepping to increase the positional addressability of a stepper motor is well-known in the art. Reference materials showing the application of microstepping include the following:
Compumotor Catalog, Step Motor & Servo Motor Systems and Controls, Parker Motion & Control, Rohnert Park, Calif.; Compumotor OEM650 Drive and Drive/Indexer User Guide. P/N 88-013157-02A. Compumotor Division, Parker Hannifin Corporation, Rohnert Park, Calif.; and Data Sheet, IM2000 High Performance Microstepping Controller. Intelligent Motion Systems, Inc., Taftville, Conn.
Patents that disclose methods for increasing the accuracy of a stepper motor in microstepping mode include:
U.S. Pat. No. 4,710,691 which discloses the use of a special apparatus to characterize positional error and correct this error by a process of measurement, adjustment, re-checking, and storing the corrected phase winding current values in memory;
U.S. Pat. No. 4,584,512 which discloses the use of harmonic frequencies of the stepper motor windings current to adjust motor resonance; and
U.S. Pat. No. 4,115,726 which discloses the use of odd harmonics for stepping motor compensation.
Selection of a lead screw thread pitch is computed based on factors that are closely coupled. Some of these factors are either fixed, or must be held within certain limits, for practical reasons. For example, the motor that drives the lead screw shaft can only operate with the needed precision over a certain range of speeds. This speed range and the need to be able to write a swath using a variable number of channels are both key factors in determining the pitch of the lead screw. In an image processing apparatus, these factors are known to restrict the possible options for lead screw thread pitch to within a very narrow range of values. As a result, the precision lead screw currently used in the image processing apparatus described above is an expensive component to manufacture and requires complex finishing operations, with ground threads to provide the needed accuracy.
One patent that discloses a method for lead screw selection is U.S. Pat. No. 5,264,949 which discloses a scanning mechanism where the lead screw pitch is specified to provide linear movement of one pixel for an integral number of stepper motor steps. It is significant to note that the apparatus disclosed in this patent does not employ microstepping mode and is limited to incremental scan head movement of a single pixel at a time. The problems addressed by the present invention are significantly more complex in scale, resolution, required accuracy, and flexibility than the problems addressed in U.S. Pat. No. 5,264,949.
Conventional approaches to the problem of precision imaging using a variable number of channels do not provide workable solutions. For example, a stepper motor can be operated only within a certain limited range of speeds. Design of a stepper motor to provide precision positioning over 30 different speeds (for using from 1 to 30 channels) would be difficult and costly. Overall, the acceleration and deceleration characteristics of motors constrain the limits for alternate motor solutions.